The ciliary rootlet is a prominent anatomical structure of the photoreceptor but neither its molecular composition nor function is known. We have recently characterized a novel protein, rootletin, that is a component of the photoreceptor ciliary rootlet. This discovery marks a major progress in this area of study and provides the scientific framework as well as technical means to begin probing the function of the ciliary rootlet. We propose that rootletin is the major constituent of the rootlet and that rootletin polymerizes into the stable filamentous structure via its extensive coiled coil domain. Rootlet is likely to interact with a number of other cytoskeletal elements including actin filaments and intermediate filaments. These interactions may be functionally important for tensile strength, positioning of organelles, and intracellular transport. Maintenance of the normal rootlet structure is therefore likely to be essential for photoreceptor function and survival and genetic defects involving rootletin may underlie some forms of human retinal degenerative diseases. These hypotheses/questions will be addressed through a combination of in vitro and in vivo studies.